The present invention relates to a key-board switching unit or, more particularly, to a novel key-board switching unit which is very versatile in design facilitating easy manufacturing by virtue of its unique and improved structure.
Various kinds of modern electronic instruments, such as a pocketable electronic calculator, are provided with a key-board switching unit for operating the instrument, for example, in calculation with the calculator by producing binary-coded signals. Conventional key-board switching units have a structure in which a printed circuit board having a fixed contact points is covered with a key-board covering pad of an electrically insulating rubbery material provided with one or a plurality of movable contact points leaving a narrow interspace therebetween by means of spacers. The binary digital signals are obtained when the movable contact point is contacted with the corresponding fixed contact point or comes apart therefrom according to pushing or releasing of the covering pad at the appropriate position.
The above mentioned printed circuit board used in the key-board switching unit is usually prepared by providing a desired electric circuit with a metal foil bonded to an insulating board and etched to have a desired circuit pattern. According to the recent trend of miniaturization of electronic instruments, the circuit pattern on the printed circuit board is also required to be finer and finer and more and more complicated. Sometimes there arises a difficulty in forming a so fine and complicated circuit only on one side of the circuit board completed as such.
Such a difficulty is usually obviated by providing a printed circuit pattern also on the bottom surface of the circuit board in addition to the top surface of the board facing the covering pad, electric connection between two circuit patterns on the top and the bottom surfaces being obtained by wiring through one or more of penetrating holes provided in the board. Alternatively, jumping connections are provided crossing over a printed circuit pattern formed as usual on the surface of the circuit board with an insulating layer therebetween.
The above mentioned means for obviating the difficulties in forming a so fine and so complicated circuit pattern on the surface of a circuit board are never free from the disadvantages of the complicated procedure for manufacturing the printed circuit boards necessarily leading to increased production costs of the desired key-board switching units.